A conventional lock includes a lock housing and a lock cylinder provided within the lock housing. The lock cylinder includes a shell for the lock cylinder, a locking hole provided in the shell into which a key can be inserted, and a locking block slidably mounted on the shell. Typically, one key can just unlock one lock. Therefore, people have to hold a group of various keys corresponding to different locks, and it is inconvenient to find out the key corresponding with a particular lock from the group of keys. If the group of keys were lost or were duplicated by other people for purpose of hostility, all the locks corresponding with the group of keys must be replaced and thus it is very inconvenient to do so. Therefore, for the conventional locks, they are impossible to be used with a new key after the original key is lost.